It has long been known to provide one or more protection devices connected to a two-wire telephone subscriber line in order to protect equipment coupled to the line from excessive voltages and currents which may occur on the line. Such excessive voltages and currents may arise from lightning strikes, power line crosses, and currents induced from adjacent power lines.
A known protection device for use at the central office end of each of the tip and ring wires of a telephone subscriber line comprises an overvoltage protector, typically comprising a carbon block or gas tube arrangement, connected between the wire and ground, and an overcurrent protector, typically comprising a so-called heat coil which consists of a resistance connected in series with the wire and a thermally sensitive shorting mechanism which is responsive to the temperature which this resistance reaches, in response to an excessive current passing through it, to provide a permanent short of the wire to ground.
Such known devices introduce various well-known disadvantages or problems. For example the heat coil is not resettable, so that it must be identified and replaced after it has effected a protective operation, and because it is a thermal device it is slow to operate. Nevertheless, such devices continue to be used extensively for protection purposes in view of their low cost and relatively small size and the lack of economic alternatives.
Various solid state devices have been proposed for protection purposes. In particular, semiconductor voltage protectors have been proposed for replacing carbon block and gas tube arrangements. Furthermore, a solid state overvoltage/overcurrent protector is known, in the form of SGS device type L3122, which includes two SCRs (semiconductor controlled rectifiers) or thyristors with respective gates which can be triggered in response to an excessive current flowing through a resistor connected in series with the respective wire of the telephone subscriber line. Although such a device automatically resets itself on termination of the excessive current, it involves disadvantages in that triggering of at least one of the two thyristors must be indirect, i.e. via an intermediate active device such as a bipolar transistor and additional resistive components. This leads to problems of sensitivity and high power dissipation in the event of low-level a.c. fault conditions.
An object of this invention, therefore, is to provide an improved solid state protection device, and an improved protection arrangement using such devices.